1. Field of the Invention
The present invention relates to catalysts for a fuel cell, methods of manufacturing the same, and fuel cells, and particularly to a catalyst having Pt or the like attached on the surface of a conductive carrier.
An oxygen-hydrogen cell is a typical example of the fuel cell. This utilizes the reverse reaction of electrolysis of water. By externally supplying oxygen as the active material of a cathode and hydrogen as the active material of an anode, electrical energy can be extracted. Other active materials of the anode are methanol, ethanol, and methane.
2. Description of the Related Art
Conventionally, fuel cells having large capacity for spaceships, small-scale power stations, and automobiles have been developed. Recently, however, there has been a growing demand for fuel cells as batteries for portable information processing apparatuses such as portable terminals, cellular phones, and notebook PCs.
Fuel cells include direct methanol fuel cells that use methanol as fuel and directly obtain H+ from the methanol and indirect methanol fuel cells that decompose methanol into hydrogen and then obtain H+ from the hydrogen. The indirect-type fuel cells require reactions at high temperatures in order to decompose methanol, and therefore, are not suitable for portable terminals. The direct-type fuel cells have a merit in that it is possible to cause reactions to proceed at room temperature.
In the direct methanol fuel cells, at a cathode and an anode, the following reactions occur at the catalyst surfaces of the respective electrodes:Anode (fuel electrode): CH3OH+H2O→CO2+6H++6e−,Cathode (air electrode): 3/2O2+6H++6e−→3H2O.Accordingly, the overall reaction is:Overall reaction: CH3OH+3/2O2→2H2O+CO2.That is, electrons obtained in this reaction, for instance, six moles of electrons, can be utilized as electrical energy.
Conventionally, a variety of catalysts have been studied in order to increase the speed of the above-described reaction. Of these, catalysts that have a variety of metals, principally platinum, carried on carbon particles or carbon substrate are used, which are specifically catalysts that have metal particulates having electrocatalytic activity, such as Pt particulates or particulates of a Pt alloy of, for instance, Pt and Ru, carried on conductive carbon particles. Rate of reaction on a catalyst surface relates directly to the amount of current, and contributes to power generation efficiency. Accordingly, catalysts having high rates of reaction, that is, catalysts having a great surface area per unit mass (specific surface area), are desired.
According to the conventional method of preparing catalysts, for instance, carbon particles are dispersed in an aqueous solution including a Pt compound. Then, an alkaline aqueous solution is dropped to reduce the Pt compound, and the carbon particles are caused to carry precipitated Pt particulates.
However, according to this method, it is possible to cause the catalytic Pt particulates to adhere to the carbon particles, but the amount is small. Accordingly, there is a problem in that the catalysis is insufficient, thus resulting in an insufficient rate of reaction in fuel cells.
In order to improve catalysis, it is desirable to cover the entire surface of the carbon particles while maintaining the size of the Pt particulates. However, a prolonged period of reduction to increase the amount of precipitated Pt particulates causes a problem in that adjacent Pt particulates on the surface of the carbon particles are coupled by newly precipitated Pt to increase particle size, thus having the opposite effect of reducing surface area to decrease catalysis.